Numerous devices exist in the prior art for detecting infiltration, that is, a condition when an infusion needle has become dislocated from a patient's vein, and fluid being delivered enters surrounding tissue instead of the vein. Many of these devices, including the device disclosed in U.S. Pat. No. 4,530,696, examine the pressure of the fluid being delivered, and subject to processing the pressure data in order to determine the rate of change of pressure under certain circumstances. The inventor is unaware, however, of any system utilizing an external device for delivering to the fluid line a negative pressure step, the response to which is monitored by a pressure-sensitive device.